docs: def is_set(self, key):
          """Check to see if the key has been set in any of the data locations.
          """
          path = key.split(self._key_delimiter)
  
          lower_case_key = key.lower()
          val = self._find(lower_case_key)
  
          if val is None:
              source = self._find(path[0].lower())
              if source is not None and isinstance(source, dict):
                  val = self._search_dict(source, path[1::])
  
          return val is not None